The decision, which will add up to £80 million to the construction costs, was taken by the Minister of Public Works, who was a guest on the 36,000-acre estate last year. Other recent visitors have included King Juan Carlos, bankers, industrialists and members of the European nobility.

Local people and environmentalists are furious and flatly dismiss the ministry’s claim that the rerouting is to protect the endangered imperial eagle and two other rare birds. That, they say, is merely an excuse for preserving “the hunting estate of a feudal lord”.

The duke, Britain’s richest man with an estimated fortune of £4.75 billion, spent £2 million two years ago on buying a ten-year lease for La Garganta, Spain’s largest private sporting estate, from the Duke of Bavaria.

The estate straddles the Castilla La Mancha-Andalucia border and is regarded as one of Western Europe’s last truly wild landscapes, a harsh but beautiful area of forests and mountains where guests can hunt deer, wild boar and red-legged partridge.

As an addition to his portfolio — which includes 300 prime acres of Belgravia and Mayfair, estates in Lancashire, Cheshire and Scotland and land and property in America, Canada, Australia and France — it cannot be beaten.

But there is a drawback. The AVE, the high-speed railway linking Madrid, the capital, with Seville, runs through the centre of La Garganta. Admittedly, the bullet-like trains do so in the blink of an eye, but the prospect of a much noisier intruder emerged last year when the Ministry of Public Works announced the construction of a toll motorway linking Toledo with Córdoba. The logical route was beside the AVE, where the greatest environmental consequences had already been incurred. The respected Spanish Society of Ornithology and the Centre for the Conservation of Nature both concurred.

The alternative route provisionally approved by the ministry will still shadow the AVE, but, when it arrives in the vicinity of the Duke of Westminster’s estate, it will make a sudden detour eastwards, skirting La Garganta and passing through the town of Fuencaliente, before curving west again to rejoin the railway line. Residents and property-owners along the proposed detour have joined local environmentalists in denouncing the decision, which will bring no benefits to the region as there will be no exit.

They observed pointedly that Francisco Álvarez Cascos, the Public Works Minister, enjoys a spot of shooting and was a guest at La Garganta last year. Sources on the estate say that the minister is expected again next month for the partridge season.

The ministry insists that its decision was made “solely for environmental reasons” to avoid disturbing the nesting areas of the imperial eagle, black stork and black vulture.

Opponents note that the new and longer route via Fuencarral passes through equally sensitive countryside that, three years ago, under European legislation, was given the twin status of specially protected zone for birds and area of community interest.

They also point out that four months after the Spanish Society of Ornithology had recommended that the motorway should run parallel with the AVE railway line, it had a change of heart, having learnt that the duke’s estate was home to “two pairs of black stork, 20 black vultures and three imperial eagles”. For groups such as the Association of the Sierra Madrona and Ecologists in Action, the presence of these protected species is no more than “an excuse”.

Francisco Diaz Buenestado, a vocal opponent, told The Times: “It doesn’t make sense and anyone would agree just by looking at the map of the two routes. Creating a new corridor which would be only 12km (7.5 miles) at its widest will produce a barrier effect for species already on their way to extinction, such as the lynx and the wolf.

“It will affect one of Europe’s most important bat refuges and endanger at least 12 rare plant species. What’s more, country rangers have found the nests of all three types of rare birds in this area, the same ones that the ministry has based its decision upon for not going through La Garganta. A lot of things do not add up and you have to conclude that this decision has been influenced by unknown pressures.”